The invention relates to a system for controlling the operation of a motor, and is more particularly of application to the screwing together of tubular components, for example, adjacent sections of tubing string or sub assemblies to the top of a tubing string.
Tubing components are conventionally screwed together using equipment which basically comprises an upper tong and a lower tong. The lower tong has a set of fixed jaws which, in use, securely grip one of the tubing components (usually the top section of a tubing string), whilst the upper tong includes a set of rotatable jaws which firmly grip the other tubing component(s).
The output of a motor, which is usually a hydraulic motor, is connected to the rotatable jaws in such a way that the motor is operable to rotate the jaws, and hence, in use, the tubing component gripped by them. The motor is used to both screw one component into the other, and to tighten them one against the other. Alternatively the motor is used to loosen the components and unscrew the components.
Problems can arise if the threads do not properly inter-engage, as a result of, for example, one of the threads being cross-threaded or of the contamination of the threads with grit. In such a situation, operation of the motor to screw the two components together may not produce an adequately secure connection, and may result in the damage, by, for example, galling of the threads.
In an attempt to reduce the risk of this happening, it is known to link the motor to a monitoring system. The monitoring system operates a device, usually a hydraulic by pass valve. A load cell is used to provide a measure of the torque being exerted, in use, on the tubing component in the upper tong. When the torque exerted passes a pre-determined maximum value, the system operates to shut down the motor.
This approach has the disadvantage that there is no automatic control of the operational speed of the motor or applied torque. Thus, if, for example, the thread on one of the components failed completely, resulting in the sudden virtually complete loss of any resistance to the tightening of the components, the motor may rotate the component held in the upper tong at a dangerously high speed.
In one broad aspect, according to the invention, there is provided a system for controlling the operation of a motor being used for the rotational assembly or dis-assembly of two members, the system comprising means monitoring, and generating a digital or analogue signal representative of the state of the two members, means relaying the representative signal in a digital form to a first processor; and means relaying the signal in an analogue form to a second processor; the first processor comprising a computer, which, in use, digitally analyses the representative signal and is so programmed as to generate a first control signal in response to the representative signal; the second processor comprising means converting the first control signal into analogue form, and means comparing it with the analogue representative signal and generating a second control signal representative of the difference between the first control signal and the representative signal; the second processor being so connected as to transmit the second control signal to the motor control means.
Such a system may be used to control a motor which is operable to rotate, for example, a section of tubing string, and the monitoring means preferably comprise means monitoring the load on the tubing by measuring the torque being exerted by the motor.
In this case, there may also, with advantage, be provided means measuring and generating signal representative of, the angular position of the tubing, the representative angular signal being transmitted, in use, to the first processor. The position measuring means may, for example, include a turn counter which, in use, generates a digital representative signal. The signal representative of angular position may be processed to give an indication of the rotational speed of the tubing being rotated.
The motor may be a hydraulic motor, in which case the motor control means may comprise an electrically actuated servo valve.
If the signal representative of the state of the two members is in an analogue form, the computer may include means converting the signal into digital form.
In another broad aspect, the invention provides a method of controlling a motor having control means operable in response to an appropriate control signal, and being used for the rotational assembly or dis-assembly of two members, the method comprising the steps of:
(a) monitoring the state of the two members and generating a signal representative of the state of the two members; PA1 (b) digitally analysing the representative signal; PA1 (c) generating the first analogue control signal in response to the analysis of the representative signal and in accordance with a pre-determined set of instructions; PA1 (d) ensuring that the representative signal is in analogue form; PA1 (e) comparing the first control signal with the representative signal and generating a second control signal corresponding to the difference between the first control signal and the representative signal; and PA1 (f) transmitting the second control signal to the motor control means. PA1 (a) monitoring the relative angular displacement of the first member; PA1 (b) generating a signal representative of the relative angular displacement of the member; PA1 (c) digitally analysing the displacement signal; and PA1 (d) providing that the first control signal is generated in response to both the representative signals. PA1 (a) monitoring the relative angular displacement between the members, and the amount of torque being exerted, as the members are rotatably tightened. PA1 (b) when the torque reaches a pre-determined level, calculating the rate of change (first differential) of torque with respect to angular displacement; and PA1 (c) determining whether or not the rate of change is of an acceptable value, i.e. a value characteristic of the case in which the maximum shoulder torque has been reached or passed.
This method may be used in a process of rotatably tightening together two members, using the motor to rotate one member relative to the other, in which the state of the member is monitored by monitoring the torque exerted by the motor on one member relative to the other, and which method includes the steps of:
The invention additionally provides a method of determining whether the maximum shoulder torque between two members being fitted together, by rotating and tightening them one against the other, has been reached or passed; the method comprising the steps of:
Such a method may include the additional step of halting the process of tightening the two members together if the rate of change is not of an acceptable value.
In the case of two tubing components which are being screwed together, the extent of movement of one component towards the other is limited by a stop formed on one of the components. Where the components are tubing sections, the stop is usually formed as a shoulder which is an integral part of the box containing the screw-threaded portion of one of the sections.
As the two sections are screwed together, the shoulder will eventually abut the end of the other section, causing a rapid increase in the resistance encountered to the relative tightening rotation movement of the sections.
On a graph of torque against relative angular displacement of the two sections this is manifested as a sudden increase in the applied torque. Opinions vary as to the precise position on the graph of the maximum shoulder torque point, but it is generally accepted that the gradient of the graph after the maximum shoulder torque is considerably greater than that of the graph before the maximum shoulder torque.
It is possible to obtain an idea of whether or not the two sections are properly connected by ascertaining whether or not the maximum shoulder torque has been reached.
Conventional attempts to do this involve the use of equipment which monitors the torque being exerted by the motor, and which determines that the maximum shoulder torque has been reached in response to a sudden increase in the torque exerted by the motor. However, the accuracy of this approach is compromised by the random variation in adjacent torque readings, such variations being caused by, for example, electrical "noise" in the torque monitoring equipment or by the thread condition.
Using the method according to the invention, the problems arising from noise can be obviated by measuring the gradient over a relatively large number of points. Furthermore, the method determines whether or not the maximum shoulder torque has been reached using a relatively easily distinguishable quantity, the gradient of the torque against Turns graph, an acceptable value of which will be considerably larger than that of an unacceptable value.